As used herein the terms "wellbore", "borehole" and "fluid passage" are intended to encompass any flow passage such as is defined by a drilled bore in an earth formation, a well casing or production conduit that is present within the drilled bore or any other pipe or tubing that defines a flow passage through which fluid, such as well fluid may flow. The term "fluid" as used herein encompasses liquids such as crude oil and water and gases such as natural gas, as well as mixtures of crude oil, water and natural gas.
Due to the plurality of fluids in a producing oil well, flow regimes for the production of petroleum fluids from wells can become extremely complex and segregated. This becomes even more acute in deviated wells for the reason that fluid phases, fluid density and the action of gravity on the well fluid can significantly influence separation of the various phases of the production fluid when the well bore or flow conduit is deviated from the vertical. The lighter density production fluid will rise to the top of the deviated wellbore and pass over the heavier density fluid. Thus, it can be quite difficult to determine the average fluid properties (phase segregation) if conventional, centralized production logging instruments are employed. In wells producing more than one phase, the phases tend to move up the well at different velocities due to the difference in densities between the phases and in some cases one or more of the phases will be moving downwardly while other phases are moving upwardly. It has been firmly established that the light-density phases of the production fluid move up the well faster than do the heavy-density phases. It has been established that the lighter phases also occupy a small cross-sectional area when this phase segregation occurs as a result of wellbore deviation angles.
Through-tubing logging instruments are limited in diameter to the size of the smallest restriction. These small instruments are traditionally run through the wellbore in such manner that the instrument and the sensors of the instrument are centralized within the wellbore, that is they are held by various means in the center of the pipe. With the instrument thus centralized, the measurement is made inside the tool body by sensors located within the instrument housing. Hence, if a centralized instrument is operated in a inclined borehole with multiple phases present, the instrument might not detect the light phase on the top of the borehole, or the heavy phase on the bottom. The phase detection that is accomplished through the use of conventional instruments can be quite inaccurate when deviated wells are logged. The purpose of this invention to measure the fluid parameters at many selected points across the borehole, rather than taking production fluid measurements in the center of the wellbore as is conventionally done. Conventional production logging instruments are normally operated in centralized manner within the borehole or well casing. When segregation in deviated wells occurs the centralized instruments do not read the average fluid composition. Rather, they tend to sense a fluid mixture that has an indicated heavier density and is thus inaccurate due to the fact that the lighter phase fluid migrates to and remains on the upper wall of the deviated wellbore. This holds true for fluid capacitance type instruments designed to determine the fraction of water in the production fluid mixture that is being produced from a well or present within the wellbore.
Another problem with centralized logging techniques utilizing tools with embedded or internal sensors involves the quality of instrument centralization. If the instrument centralizers used in highly deviated wells do not provide sufficient force to properly overcome the weight of the instrument housing and its contents and to centralize the instrument, the instrument will tend to be decentralized by its own weight and will rest on or near the bottom wall surface of the wellbore. This leads to the sensor of the instrument being positioned in the heavy phase side of the deviated wellbore and the measurements taken to be erroneous with the heavy phase being dominant.
The problem lies in the fact that a conventional production logging tool typically measures a local internal fluid sample in deviated wells and does not measure the fluid across the whole cross-section of the wellbore. Light phases that migrate to the top wall of the well are not measured by the internal sensors of the conventional centralized instrument. The advantage of the across-the-borehole type production logging devices according to the present invention is that these instruments, using sensors that are placed in a manner to measure from one side of the borehole to the other, can accomplish a true measurement that is representative of the actual production fluid mixture. This measurement or measurements includes all of the phases that are present in the fluid mixture. It is desirable, therefore that a production logging instrument be provided having sensors which measure a combination of the light phases that are present at the top wall of the deviated wellbore and the heavier phase or phases that are located at or near the bottom wall of the wellbore. These measurements are then true representations of the various phases that might be present in the production fluid; the measurements can be efficiently processed to accurately depict the character of the well fluid flowing or present within the wellbore. Additionally, because the instrument of this invention is run decentralized, the heavier body of the tool will be positioned by the influence of gravity in contact with the bottom wall of the wellbore thus, as a consequence, positioning the lighter weight sensor arm of the tool in contact with the top wall of the wellbore. As wellbore deviation is encountered by the tool, the influence of gravity will cause it to be automatically oriented with the tool body in engagement with the lowermost wall of the wellbore or casing and with the sensor arm in engagement with the uppermost wall. This tool therefore obviates the need for rigid centralization of the tool within the wellbore according to conventional practices and thus overcome the disadvantages associated with conventional centralized production logging instruments.